Cal. Pub. Util. Code § 492

The names of the several public utilities which are parties to any joint tariff, rate, contract, or classification shall be specified in the schedule. Unless otherwise ordered by the commission, a schedule showing such joint tariff, rate, contract, or classification need be filed with the commission by only one of the parties to it, if there is also filed with the commission, in such form as it may require, a concurrence therein by each of the other parties thereto.